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(a) The geometry of the pulley structure used during the FDTD optimization of the coupling gap. Optimization parameters are the coupling gap g and the output waveguide width w770. The width of the ring resonator is kept constant. (b) A SEM image of the coupling region of a pulley structure. (c) The calculated power transfer from the ring into the waveguide in dependence of the coupling length at fixed coupling gap. The blue markers represent FDTD simulation results whereas the red line is a sinusoidal fit to the data. (d) The calculated coupling length in dependence of coupling gap. Markers are FDTD results and the red line is the linear fit.

(a) The measurement setup used to characterize the devices. Light from tunable laser sources is coupled into the chip using an optical fiber array. Transmitted NIR and visible light is detected with an InGaAs photoreceiver and a Si photodetector, respectively. (b) The optical transmission spectrum of a critically coupled ring resonator with high extinction ratio of 30 dB for 1550 nm light input. In this case the measured optical Q is on the order of 100,000. (c) The spectrum for an undercoupled ring resonator. The Lorentzian fit to the resonance dip (solid red line) reveals a linewidth of 3.5 pm corresponding to optical Q of 440,000.

(a) The measured optical quality factor for critically coupled resonator devices in dependence of waveguide width. A monotonic increase in Q is observed for waveguide widths below 900 nm, which is a result of the waveguide width (630 nm) being closer to the cutoff width for 1550 nm input light. (b) The measured extinction ratio in dependence of coupling gap and waveguide width. For the 900 nm wide waveguide the critical coupling gap is the largest (650 nm), indicating that internal losses in the ring are minimized.

(a) The TE-polarized transmission as a function of wavelength for a ring resonator with R = 40 µm. Inset: resonance at 774.24 nm. The fitted linewidth (red line) is 43 pm, corresponding to a Q of 18,000. (b) The zoom-in spectrum of an undercoupled ring resonator. The fitted linewidth (red line) is 26 pm, corresponding to a Q of 30,000.